I've built several rifles on the vesper actions. They are excellent for the 338 Lapua/Norma size of case. Like stated above you need a 1.25 barrel shank at a min.
The deep bolt flutes on the vesper doesn't make it as "smooth" as the other bats in my experience. But it's still plenty smooth for what it is, but it is noticeable if ran next to a bat with standard fluting.
For a magnum size case, the defiance is very nice also. Defiance are some of the straightest actions I've had in the shop and they do a very good job with ignition. They are definitely benchrest quality. I have no reservations using either.
So the choice would come down to your requirements….
If you need a lapua bolt face, or the cfe9 mag box I'd choose the vesper.
If going magnum bolt face and cip length mags, I'd go with a defiance, and save $200